Added manual installation instruction for Centos7 and note about firewalld.

## Documentation
Added a note about firewalld.
Added manual installation instructions.

## Init service (systemd)
Ensure the docker daemon is started after firewalld.
Added "install" directive to docker service for systemd.

Signed-off-by: Nicolas Goy <kuon@goyman.com>
Re: fetching latest binary, we can perhaps just link to the [Binaries](https://docs.docker.com/v1.1/installation/binaries/) page instead of duplicating the step.

Re: systemd stuff, I think manual installation should also be done to the same dir as that used by the rpm, which is `/usr/lib/systemd/system`. That is because the service and socket files installed by the RPM quite often have some customizations which won't go upstream, but which are desired by the users/package guidelines, and if people already have stuff in `/etc/systemd/system` (which overrides all other locations) and are under the impression their system is using files in `/usr/lib/systemd/system`, they might have a hard time figuring things out.